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The holotype specimen of the brontothere Menodus giganteus Pomel, 1849, long presumed lost, is in the Smithsonian collection. The validity of Menodus giganteus depends on an ultimate understanding of Chadronian brontothere species-level diversity. Although Chadronian brontotheres have historically been split into a multitude of species (N= 47, by 1929), based primarily on horn variation, a preliminary analysis of Chadronian brontothere morphometric data reveals that most of the variation in their horns can be explained by sexual dimorphism. We are not able to objectively group Chadronian brontothere specimens into discrete morphological units with our morphometric data. Variation is apparently continuous among those variables that were commonly used to diagnose Chadronian brontothere species. Chadronian brontotheres can be unambiguously grouped into no more than two discrete morphological units, a group with unbifurcated horns that includes the vast majority of specimens, and a much smaller group with bifurcated horns. If these groups represent valid taxa, Menodus giganteus is a nomen dubium. Megacerops coloradensis Leidy, 1870 is the earliest Chadronian brontothere name whose holotype includes a pair of unbifurcated horns, and is therefore the valid name for the more common species. The rare species with bifurcated horns is Megacerops kuwagatarhinus Mader and Alexander, 1995.